1. Field of the Invention
This invention relates to the field of parameters for describing object animation, particularly to methods of reducing the number of graphic articulation parameters (GAPs) that must be conveyed to render an object.
2. Description of the Related Art
The existing and developing Motion Picture Expert Group (MPEG) standards provide techniques for coding digital video signals over band limited channels. Included within the MPEG standards are parameter definitions which are to be used to describe the animation of 3-D objects, i.e., the movement of selected points of an image of a 3-D object from one video frame to the next. For example, graphic articulation parameters (GAPs) known as "body animation parameters" (BAPs) have been defined for describing the animation of the human body, and a set of "facial animation parameters" (FAPs) have been developed for describing the movements of a human face.
The MPEG-4 standard under development will include the capability to generate and transmit synthetic "talking head" video for use in multimedia communication systems, and will use the FAP set to convey facial animation information. The FAP set enables model-based coding of natural or synthetic talking head sequences and allows intelligible reproduction of facial expressions, emotions and speech pronunciations at the receiver. Currently, the FAP set contains 68 parameters that define the shape deformation or movements of a face. For example, the parameter open.sub.-- jaw defines the displacement of the jaw in the vertical direction while the parameter head.sub.-- yaw specifies the rotational yaw angle of the head from the top of the spine. All the FAPs are defined with respect to a neutral face and expressed in a local coordinate system fixed on the face.
The digitizing of video information typically produces very large amounts of data, which requires vast amounts of storage capacity if the data is to be stored, on a hard drive, CD-ROM, or DVD disc, for example. Similarly, transmitting the video data over a distance via some type of communications link requires a considerable amount of bandwidth. For example, the 68 parameters of the FAP set are defined as having 10 bits each. State-of-the-art modems provide 56 kbits/sec downstream capability from a central location to a home. Since the 68 FAPs represented by 10 bits at a 30 Hz video rate require only 20.4 kbits/sec, it is possible to transmit them uncoded and thus preserve their visual quality. However, this approach does not contemplate nor will it support the simultaneous transmission of multiple talking heads as part of a single video signal as may occur in a virtual meeting, for example, or the transmission of the FAPs as part of larger synthetic objects, for example, full-body animation.
To conserve storage space and to permit the use of currently available communications links, methods of reducing the amount of data required to animate an object are needed.